A new model is presented to elucidate the optical-ultraviolet ``flickering'' variability in cataclysmic variables. In this model, light fluctuations are produced by occasional flarelike events and subsequent avalanche flow in the accretion disk atmospheres. Flares are assumed to be ignited when the mass density exceeds a critical density. The disk then evolves to and stays at a self-organized critical state, in which seemingly chaotic fluctuations can be produced. We calculated fluctuating light curves by cellular-automaton simulations. The resultant power spectral density (PSD) consists of a flat part [S(f) ~ f 0, where f is frequency] at lower frequencies and a steep-decline part [S(f) ~ f -β, with β ~= 1-2] at higher frequencies. It is important that the PSD is rather insensitive to wavelength.